Common practice for designing a voltage source converter for high voltage applications involves series connected switches. Each switch has the function of opening and closing an electric circuit. For series connection the closing operation is vital since otherwise the whole valve will be an opening circuit and the valve could not operate. Hence if a switch experiences a malfunction it must be capable of achieving a short circuit failure mode. The switch normally comprises an extinguishable semiconductor element. However, most such semiconducting elements stay open in a failure mode. Hence, a converter comprising a plurality of series connected semiconducting element each semiconducting element must involve a short circuit failure mode. This may either be accomplished by a parallel circuit to the semiconducting element or by designing the semiconducting element to assume a short circuit in failure mode.
Each semiconducting element comprises an individual voltage rating for an allowable dc voltage stress across its main terminals. Hence, to design a voltage source converter for a specific total voltage rating a specific number of active semiconducting elements in series connection is demanded. If one of these semiconducting elements experiences a malfunction the whole converter trips and must be brought out of operation. To account for a semiconducting element failure it is known to arrange a plurality of redundant semiconducting elements in a string of series connected semiconducting elements. Thus the string of semiconducting elements comprises a specific number of active semiconducting elements, necessary for the operation, and a specific number of redundant semiconducting elements. When all of these redundant semiconducting elements have broken down the operation of the converter is accomplished by the still active semiconducting elements. But when one of the active semiconducting elements assumes a short circuit failure mode the converter trips and must be taken out of operation.
If thus a semiconductor failure occurs during operation, the converter remains in operation as long as the number of broken positions in one valve is less than the number of redundant positions. Depending on the system design and the converter voltage rating, several positions can be redundant. Should all redundant positions in one valve be broken and an additional position breaks down, the plant has to trip. This means that the plant will be out of operation until it has been repaired with a new set of semiconducting elements.